1. Field of the Invention
The present invention relates to a drilling tool having a cutting tool provided at a distal end of a tool body for use in drilling a workpiece, and to a throw-away tip suitable for use in drilling work.
The present invention claims priority to Japanese Patent Applications HEI 10-274000, filed on Sep. 28, 1998, and HEI 11-045482, filed on Feb. 23, 1999, and these applications are hereby incorporated by reference.
2. Description of the Related Art
As a drilling tool of this type, the inventors of the present invention propose, for example, in Japanese Unexamined Patent Publication No. 10-118820, a drilling tool in which substantially triangular planar chips are mounted respectively on distal ends of wall surfaces of a pair of chip discharge grooves facing in the direction of rotation of a tool, the pair of chip discharge grooves are formed in an outer periphery of the distal end of a tool body rotated around its axis, a pair of cutting edges whose inner peripheral ends are spaced from the axis toward the outer periphery of the tool are provided, a recess recessed between the inner peripheral ends of the cutting edges toward the rear end of the tool along the axis is formed in the center of the distal end of the tool body, and the bottom surface of the recess is formed into a shape of an inclined surface inclined toward the axis. In such a drilling tool, the inner peripheral ends of the cutting edges are spaced from the axis of the tool body toward the outer periphery of the tool, whereby a core of a workpiece will grow along the axis during drilling. However, the core can be guided by the inclined bottom surface of the recess, and can be securely discharged to the chip discharge grooves.
Incidentally, not only in drilling using a drilling tool in which the core grows during drilling, but also with a common twist drill, a site of a drilling is at a bottom of a closed hole, so that it is important to securely curl and smoothly treat chips produced by the cutting edges at the site of drilling during drilling. In this case, however, the distance is short between the end of the cutting edge adjacent to the inner periphery of the tool, that is, in the vicinity of the center of the distal end of the tool body and the axis to be the center of rotation of the tool during drilling, cutting speed is lowered, drilling form is such that the cutting edge crushes and picks up the workpiece rather than cutting, and chips are formed in sheared shapes instead of flowing shapes that are difficult to curl, so that chip packing easily occurs. Moreover, particularly in the drilling tool in which the core grows during drilling as disclosed in the above Japanese Unexamined Patent Publication, the core is discharged toward the center of the distal end of the tool body where chip packing easily occurs, so that a smooth chip treatment may be further obstructed.
In addition, as the throw-away drilling tool in which tips are detachably mounted as described above, a drilling tool disclosed in, for example, Japanese Unexamined Patent Publication No. 4-269108, Japanese Unexamined Utility Model Publication No. 1-81210, and Japanese Unexamined Patent Publication Nos. 9-262711 and 9-295212 are also known. That is, in the drilling tool disclosed in the above Japanese Unexamined Patent and Utility Model Publications, a pair of chip discharge grooves is formed in a distal end of a tool body to be rotated around its axis on both sides of the axis of the tool body, and substantially triangular planar tips are mounted on tip-mounting seats formed on the chip discharge grooves adjacent to the distal end of the tool such that one of the triangular surfaces is located as a rake face to face in the direction of rotation of the tool and the other one triangular surface is seated as a seat face on the bottom surface of the tip-mounting seat facing in the direction of rotation of the tool, one of the side surfaces disposed around the triangular surfaces is located as an end flank face to face the distal end of the tool, and the remaining two side surfaces are brought into abutment with wall surfaces of the tip-mounting seats facing the outer periphery of the tool and the inner periphery of the tool, respectively. A cutting edge is formed along a crossing ridge between the rake face and the end flank face, and the cutting edge is disposed in such a manner that the inner peripheral end thereof is located in the vicinity of the axis at the distal end of the tool, that is, in the vicinity of the center of rotation of the tool, and the cutting edge is inclined toward the rear end of the tool as it proceeds toward the outer periphery of the tool.
In the drilling tool disclosed in these Japanese Unexamined Patent and Utility Model Publications, the rake face of the tip is disposed such that it is in parallel with the axis of the tool body, or it is inclined in the direction of rotation of the tool as it proceeds toward the rear end of the tool, and an axial rake angle thereof is set to 0.degree. or as a negative angle. For this reason, the cutting resistance during drilling is large, a large driving force is required for rotationally driving the tool body, and chatter or vibration is likely to occur in the tool body, resulting in deterioration of the precision of a drilled hole, such as an increase in an enlarged margin.
On the other hand, in order to solve such problems, when the rake face is disposed so as to be inclined rearward in the direction of rotation of the tool as it proceeds toward the rear end of the tool so that a positive rake angle is formed, the wall surface of the tip-mounting seat facing the outer periphery of the tool is also inclined rearward in the direction of rotation of the tool as it proceeds toward the rear end of the tool, and consequently, a thin wall is formed between the wall surfaces of both tip-mounting seats provided at the distal ends of the pair of chip discharge grooves, that is, at a core diameter portion of the distal end of the tool body, along the axis of the tool body. When such a thin wall is formed at the core diameter portion of the distal end of the tool body, the strength of the tool body at the wall may be greatly impaired and the tool body may be damaged during drilling, resulting in shortened life of the tool.
Furthermore, in the tips disclosed in the above Japanese Unexamined Patent and Utility Model Publications, a breaker groove for curling and treating chips produced during drilling is formed on an edge of the rake face adjacent to the cutting edge. In the tips, however, since the breaker groove is a narrow groove formed on the edge of the rake face adjacent to the cutting edge as described above, when slightly elongated chips are produced according to a drilling condition, the chips pass over the breaker groove so that they cannot be securely curled. In addition, even if the chips are brought into sliding contact with the bottom surface of the groove without passing over the breaker groove, in order to sufficiently curl the chips by the narrow breaker groove, a large resistance must be given to the chips by, for example, reducing the radius of curvature of the bottom surface of the groove to suddenly change the outflow direction of the chips, resulting in an increase in the cutting resistance received by the tool body during drilling.
The present invention has been achieved in view of the background set forth above. Firstly, an object of the present invention is to provide a drilling tool capable of increasing chip treatment ability particularly in the vicinity of the center of the distal end of the tool body, and capable of preventing the occurrence of chip packing even in a drilling tool in which a core grows during drilling. Secondly, another object of the present invention is to provide a drilling tool including tips mounted to a pair of tip-mounting seats, formed on a distal end of a tool body on both sides of an axis, capable of reducing cutting resistance during drilling to decrease rotational driving force of the tool body while ensuring the strength of the distal end of the tool body at its core diameter portion, and which is capable of increasing the precision of a drilled hole by suppressing chatter and vibration. Thirdly, another object of the present invention is to provide a tip, particularly for use in drilling, capable of curling chips securely, and capable of reducing the cutting resistance at the time of drilling.